Click on the ruler icon located in the toolbar. Choose from a line, a ruler or an arrow then take the cursor to the diagram click and drag to place a line.
Edit the length of the line or label an arrow in the panel.
Tip: Use the Shift key while dragging to have a line snap to a cardinal direction.
Ruler lines may be anchored to to objects on the diagram.
Measurement Guides are a helpful way to see the distance between tables. Measurement guides will appear when selecting a table and moving it towards another table.